Large diameter (up to 300 mm), thin (0.5 mm and less) transparent wafers are becoming increasingly important in modem semiconductor and biomedical device processing and manufacture. Moreover, these large, thin, transparent wafers require surfaces that are flat to similar tolerances as those found in polished silicon wafers traditionally used in semiconductor processing and manufacture, i.e., the wafer surface must be flat to better than a tenth of the wavelength of visible light. Such surface flatness is traditionally characterized as flat to λ/10, where the wavelength λ is approximately 0.5 μm. This degree of surface flatness may be measured using an interferometer. This is a well known type of instrument in which a surface is compared with a reference surface by observing the patterns of interference fringes when the surfaces are brought in close proximity, either virtually or in reality, under appropriate illumination. These fringes are formed as wave fronts of light reflected from each surface constructively and destructively interfere with each other, enabling measurements of differences in separation of a fraction a wavelength.
FIG. 1 shows a traditional industrial interferometer 10, referred to as a Newton Interferometer, that is widely used to interferometrically test large optical components such as telescope mirrors. The operation of a Newton Interferometer is described in detail, for instance, on pages 1-18 of “Optical Shop Testing”, second edition, edited by Daniel Malacara and published in 1992 by John Wiley & Sons, Inc. of Hoboken, N.J., the contents of which are hereby incorporated by reference.
A Newton Interferometer uses an essentially monochromatic light source 12, such as a low-pressure sodium vapor lamp emitting yellow light at 589 nm. This is passed through a diffuser 14 and a beam-splitter 18 to illuminate the optical surface to be tested 22 that is in close proximity to a reference flat 20. Because sodium has a coherence length on the order of millimeters, an observer 24 looking at the surface to be tested 22 via its reflection of the semi-reflecting surface 18 will see a series of light and dark fringes. These fringes are interference fringes. A light fringe is caused by constructive interference in which the optical path difference between a ray reflected from the surface to be tested 22 and a ray reflected from the reference surface 20 is an integer number of the wavelength of the illuminating light. A dark fringe is caused by destructive interference in which the path difference is a half a wavelength of the illuminating light, or an integer multiple thereof. It can be shown that the location of a light fringe at a first point and an adjacent dark fringe at a second point indicates a difference in separation between the reference surface 20 and the surface being tested 22 of λ/4, or 147.5 nm at the two points.
The problem of using a Newton Interferometer for testing large thin wafers is that such wafers deform under gravity when oriented horizontally. In order to test a 300 mm wafer, a reference flat of larger diameter having a surface flatness of λ/20 would be required. The problem, however, is that if a 0.5 mm thick wafer that has a curved surface is placed on top of such a reference flat, it would simply bend and conform to the surface. So all large thin wafers tested in a Newton Interferometer would have distortions due to gravity and may even appear to have flat surfaces, even though they may have local curvatures and other deformities.
While many other interferometers are known, none is well suited to the task of easily and cheaply measuring the flatness of a large diameter (up to 300 mm), thin (0.5 mm and less) transparent wafer to the accuracies required.
For instance, U.S. Pat. No. 6,744,522 describes an interferometer for measuring the thickness profile of thin transparent substrates. This patent describes a method in which a selected location on the object is chosen, and an interference pattern is obtained at a first wave-length. This pattern is used to obtain a first estimate of optical thickness. The procedure is repeated at the same location using a second wavelength to obtain a second estimate of the optical thickness. A third estimate of the optical thickness is then calculated by combining the first and second estimates. This process allows the various errors inherent in the measurements, including the effects of multiple reflections from both surfaces of the sample, to be reduced. The process is lengthy and complicated, and this patent serves to emphasize the problem of making thickness profile measurements on thin transparent samples.
U.S. Pat. No. 5,694,217 describes an interferometer used for testing optical surfaces and the stress and strain within a thin specimen. It is an example of the application of polarized waves using λ/4 plates. The use of polarized light for the measurement and the need to extract the results from experimental data make this approach too complicated for high-volume measurements of large components.
U.S. Pat. No. 5,054,925 describes a method for aligning an interferometer.
U.S. Pat. No. 6,972,850 describes an apparatus and method for measuring the shape of the optical surface using interferometer. Both the light reflected by the reference surface and the light reflected by the surface to be measured are made to interfere. This method is intended for curved surface measurements, and the procedure is complicated.
U.S. Pat. No. 6,381,015 describes an inspection apparatus directed to biomedical analysis. The apparatus includes an optical interferometer, a phase modulator and a photo detector. The spectrum of the electrical signal contains information about an illuminated sample. This equipment is expensive.
U.S. Pat. No. 5,986,759 describes an optical interferometer with two optical outputs that produces two linearly independent signals, allowing high quality measurements to be made. This equipment is, however, expensive and complex to operate.
The review of patents demonstrates that it is desirable to provide an interferometer system that is capable of measuring precisely and efficiently the flatness of thin, transparent wafers with large diameters and the profile of wafer surfaces to satisfy modern industrial needs.